1. Field of the Invention
The invention relates generally to the field of marine seismic survey systems. More particularly, the invention relates to marine seismic systems deployed on the floor or bottom of a body of water.
2. Background Art
Marine seismic survey systems known in the art include so-called “ocean bottom cables” (OBCs). OBCs are deployed on the bottom of the ocean or other body of water, beneath which it is desired to determine the geologic structure and rock characteristics. A typical OBC includes a plurality of seismic sensors at spaced apart locations along a cable. One or more seismic energy sources are towed in the water by one or more vessels, and are periodically actuated. Seismic signals detected by the sensors in the OBC are recorded, typically by a recording unit forming part of the OBC.
The sensors in an OBC typically include motion sensors, such as geophones or accelerometers. The motion sensors are typically packaged in such a way that the motion sensors are disposed along different sensitive directions. “Sensitive direction” means the direction along which the particular motion sensor is most sensitive to movement. As is known in the art, typical motion sensors are primarily sensitive to movement along a “principal axis” or principal direction, and are substantially insensitive to movement along any other direction. Having such motion sensors provides the OBC with the capacity to detect seismic energy propagating along a plurality of directions and to resolve the direction of such movement.
Typical OBCs also include pressure sensors or pressure gradient sensors, such as hydrophones, at spaced apart locations along the cable. Hydrophones generate a signal proportional to pressure change in the surrounding medium (the water). Having a pressure change signal related to seismic energy propagation, combined with motion signals along different sensitive directions, enables using many different techniques known in the art for interpretation of the signals so as to reduce the effects of acquisition artifacts such as water bottom multiple reflections and water layer multiple reflections. The multiple direction motion signals also enable interpretation of converted wave (compressional to shear) seismic signals, for determining properties of the subsurface earth formations such as amplitude versus offset (AVO) and directional anisotropy.
A number of “deghosting” and water layer effect attenuation techniques are known in the art for use with OBCs. One such technique is described in U.S. Pat. No. 4,486,865 issued to Ruehle. Pairs of detectors each comprise a geophone and a hydrophone. A filter is applied to the output of at least one of the geophone or hydrophone in each pair so that the frequency content of the filtered signal is adjusted. The adjustment to the frequency content is such that when the filtered signal is combined with the signal from the other sensor, the ghost reflections cancel.
U.S. Pat. No. 5,621,700 issued to Moldovenu also discloses using at least one pair of sensors in a method for attenuating ghosts and water layer reverberations.
U.S. Pat. No. 4,935,903 issued to Sanders et al. discloses a method for reducing the effects of water later reverberations which includes measuring pressure at vertically spaced apart depths, or by measuring pressure and particle motion using sensor pairs. The method includes enhancing primary reflection data for use in pre-stack processing by adding ghost data.
U.S. Pat. No. 4,979,150 discloses a method for marine seismic exploration in which output of substantially collocated hydrophones and geophones are subjected to a scale factor. The collocated hydrophones and geophones can be positioned at the sea floor or above the sea floor.
The foregoing description is intended to emphasize the potential benefits of seismic surveys acquired using OBCs. A limitation to using OBCs is that it takes a substantial amount of time to deploy OBCs, and specialized handling equipment is typically required to extend the OBC from a deployment vessel, and place the OBC on the water bottom. After deployment, it is often necessary to determine the exact position on the sea floor at which each sensor in the OBC ultimately comes to rest on the water bottom, because currents in the water, and viscous effects on the various components of the OBC may cause some of the sensors to come to rest at a different location than the location at the water surface of each sensor when the OBC was extended from the deployment vessel. It is also necessary to retrieve the OBC to access a recording device coupled to the OBC in order to make use of the signals generated by each of the sensors in the OBC. To survey a substantial geographic area using OBCs thus requires a number of deployments and retrievals of the OBCs used in any survey operation.
The need for repeated deployment and retrieval of OBCs has made it necessary for the various mechanical load handling components in a typical OBC to withstand repeated applications of axial stress along the OBC cable and along interconnecting devices that couple the various components of the OBC. It is also necessary for the various components of an OBC to withstand immersion in water, sometimes to substantial depth (as much as 1,000 meters). While it is well known in the art how to form cables, sensor enclosures and interconnecting devices for OBCs to withstand environmental and operational stresses such as the foregoing, the devices known in the art are frequently heavy, cumbersome and expensive to manufacture.
It is also known in the art to use modified versions of typical seismic sensing equipment intended for use on dry land, however, such modified land systems are typically suitable only for relatively shallow water depths (15 to 30 meters). There is a need for inexpensive to manufacture, rugged OBCs that are suitable for use in greater water depths than modified land-based sensor systems, and that are easier to deploy and retrieve than typical OBCs.